Context Patterns in Haskell
Markus Mohnen

Lehrstuhl für Informatik II, RWTH Aachen, Germany
mohnen@informatik.rwth-aachen.de

Abstract. In modern functional languages, pattern matching is used to
define functions or expressions by performing an analysis of the structure
of values. We extend Haskell with a new non-local form of patterns
called context patterns, which allow the matching of subterms without
fixed distance from the root of the whole term. The semantics of context
patterns is defined by transforming them to standard Haskell programs.
Typical applications of context patterns are functions which search a
data structure and possibly transform it.
This concept can easily be adopted for other languages using pattern
matching like ML or Clean.